During the process of manufacturing semiconductors, resist-covered semiconductor wafers are placed into a wafer cassette and subject to megasonic sulfuric acid cleaning. As this cleaning progresses, air bubbles start to form on the front surfaces of the wafers. The air bubbles would continue to form until the adhesion of the bubbles on the surfaces would cause random wafers to float out of the cassette to the top of the acid bath. These random wafers would be improperly cleaned or damaged due to microscratching by particles which also floated to the top of the acid bath.
Investigation of this phenomenon determined that it was caused by extra thick resist on the random wafers. The obvious solution was to standardize the thickness of the resist.
For many years, the only solution to this problem has been to thin the resist to a predetermined thickness. Although it increased the cycle time and required additional handling, the additional process of plasma stripping of excess resist has been a standard operation before acid cleaning. Since the silicon wafers are extremely fragile, no system of clamping the wafers into place in the wafer cassettes was considered workable.